GaN-based compound semiconductor materials have drawn an increasing attention in recent years as semiconductor materials for short wavelength light-emitting devices. The GaN-based compound semiconductor is fabricated by using a sapphire single crystal, various other oxides and Group III-V compounds as a substrate and by applying an organic metal vapor phase chemical reaction method (MOCVD method) or a molecular beam epitaxy method onto the substrate.
One of the characteristic properties of GaN-based semiconductor materials is that current diffusion is small in a transverse direction. Therefore, a current can be injected only into the semiconductor immediately below electrodes and light emitted from a light-emitting layer is blocked by the electrodes and cannot be output. Therefore, a transparent electrode is ordinarily used as a positive electrode in the light-emitting devices of this type, and light is output through the positive electrode.
The positive electrode according to the prior art has a layer structure formed by combining an oxide of Ni or Co with Au as a contact metal in contact with a p-type semiconductor (for an example, refer to Japanese Patent No. 2,803,742). The film thickness of the contact metal has been minimized in recent years by using a metal oxide having higher electric conductivity such as ITO, and a layer structure that improves transparency without using a contact metal has been employed as the positive electrode (for an example, refer to Japanese Unexamined Utility Model Publication No. 6-38265).
The layer of the electrically conductive transparent material such as ITO has higher transmissivity for light than the layer of the oxide such as Ni or Co and can be formed into a relatively large thickness. Whereas the oxide of Ni or Co has a film thickness of from about 10 to about 50 nm, a thickness of 200 nm to 500 nm is utilized for the layer of ITO, etc. The conductive transparent material of an n-type such as ITO can improve light output efficiency owing to its high light transmissivity in comparison with Ni and Co. However, the problem remains in that a high contact resistance occurs with a p-type semiconductor because the material is n-type.